Exhaust gases from diesel engines contain fine particles (particulate matter) mostly comprising carbon (soot, etc.) and high-boiling-point hydrocarbons, which are likely to adversely affect humans and the environment when discharged to the air. Accordingly, a ceramic honeycomb filter for removing the particulate matter to clean the exhaust gas, which may be called “honeycomb filter” below, has conventionally been mounted in an exhaust pipe connected to a diesel engine. As shown in FIGS. 1 (a) and 1(b), a honeycomb filter 20 comprises a ceramic honeycomb structure 10 comprising porous cell walls 2 forming large numbers of flow paths 3, 4 and a peripheral wall 1, which may be called “honeycomb structure” below, and plugs 6a, 6b alternately sealing both ends 8, 9 of the flow paths 3, 4 in a checkerboard pattern. The exhaust gas enters the outlet-sealed flow paths 3 and exits from the inlet-sealed flow paths 4 through the cell walls 2 as shown by dotted arrows. While the exhaust gas passes through pores on and in the cell walls, the particulate matter is captured by pores on and in the cell walls.
As the cell walls 2 continues capturing particulate matter, their pores are clogged, resulting in increased pressure loss. A honeycomb filter can be regenerated by burning the accumulated particulate matter by a burner, a heater, etc. Alternatively, particulate matter is brown off by washing air and burned to regenerate the honeycomb filter. Recently conducted are a method of continuously burning particulate matter captured by pores (not shown) in cell walls 2 of a honeycomb filter 20 with a catalytic material comprising a platinum-group metal catalyst and a high-specific-surface-area material such as active alumina, which is carried on the cell walls 2 or in the pores, or a method of spraying an unburned fuel into an exhaust gas upstream of a honeycomb filter and burning it, thereby burning the captured particulate matter, etc.
The honeycomb filter 20 is required to have a high particulate-matter-capturing efficiency with a long capturing time and low pressure loss. Among them, the capturing time (time during which particulate matter can be captured at a certain pressure loss or less) is preferably as long as possible. A short capturing time necessitates the captured particulate matter to be burned frequently by heating with an electric heater or a burner, or by burning an injected fuel, resulting in much heating energy consumption, and poor fuel efficiency of diesel engines.
JP 7-163823 A discloses a porous ceramic honeycomb filter comprising cell walls having porosity of 45-60%, the relation between the specific surface area M (m2/g) of all pores penetrating from their surfaces to their inside and the surface roughness N (μm) of the filter being 1000M+85N ≧530. It describes that such shape extends the capturing time, thereby reducing the number of regeneration operations. It further describes that when fine talc particles and coarse silica particles are used in combination, the coarse silica particles located on the surface increases the surface roughness N, and the fine talc particles increases small pores as a whole, thereby increasing the specific surface area M of pores. However, it is still insufficient to the problem of a short capturing time. Particularly because particulate matter in the exhaust gas has become smaller and more uniform due to the improvement of diesel engines (high-pressure fuel injection) recently, the accumulated particulate matter increases the pressure loss more, so that the capturing time tends to become shorter.
JP 8-931 A discloses a honeycomb filter having porosity of 40-55%, an average pore diameter of 5-50 μm, and a surface valley level (surface pore area ratio in a portion lower than an average surface determined by surface roughness measurement) of 20% or less. It describes that because particulate matter captured on the filter surface is easily detached, this filter has good regeneration efficiency with washing air, suffering less pressure loss increase even after a long period of use. Because the honeycomb filter described in JP 8-931 A has a small portion lower than the average surface determined by the roughness of the surface on which particulate matter is predominantly captured, the captured particulate matter is well detached from the honeycomb filter by washing. However, because this honeycomb filter has a small portion in which particulate matter is captured, the accumulated particulate matter tends to drastically increase the pressure loss. Particularly because particulate matter in the exhaust gas has become smaller and more uniform due to the improvement of diesel engines (high-pressure fuel injection) recently, particulate matter accumulated in the honeycomb filter increases the pressure loss more, so that the capturing time becomes shorter.
WO 01/15877 describes that when the porosity was increased to change the valley level from 15% to 30% in a particulate-matter-removing filter comprising a cordierite honeycomb structure having a cell wall thickness of 0.3 mm and a cell density of 200 cpsi, the capturing ratio was improved by about 5% in a usual flat-wall honeycomb structure, and about 10% in a wave-wall honeycomb structure. However, the valley level of about 30% is still insufficient to increase the capturing time, and a further increase in the porosity lowers the strength of the honeycomb structure. Accordingly, it is difficult to provide a honeycomb filter with long capturing time, high strength, and low pressure loss.